As is well known, olefins, or alkenes, are a homologous series of hydrocarbon compounds characterized by having a double bond of four shared electrons between two carbon atoms. The simplest member of the series, ethylene, is the largest volume organic chemical produced today. Importantly, olefins including ethylene, propylene and smaller amounts of butadiene, are converted to a multitude of intermediate and end products on a large scale, mainly polymeric materials.
Commercial production of olefins is accomplished by various methods including fluid catalytic cracking of hydrocarbons, stream cracking of hydrocarbons, e.g. alkanes, as well as dehydrogenation of alkanes, among other processes. Steam cracking of hydrocarbons is carried out using a feed which is ethane, propane or a hydrocarbon liquid ranging in boiling point from light straight-run gasoline through gas oil. Ethane, propane, liquid naphthas, or mixtures thereof are a preferred feed to a hydrocarbon cracking unit. Hydrocarbon cracking is generally carried out thermally in the presence of dilution steam in large cracking furnaces. Reaction conditions for steam cracking are selected to maximize the production of light olefins. Typically, cracking is practiced at a weight ratio of 0.3:1.0 of steam to hydrocarbon with the reactor outlet at 760°-870° C., and slightly above 100 kPa (atmospheric) pressure.
The type of feedstocks and the reaction conditions determine the mix of products produced. Many steam crackers operate on light paraffin feeds consisting of ethane and propane and the like. However, a significant amount of steam cracking capacity operates on feedstocks which contain propane and heavier compounds. Steam cracking such feedstocks tends to produce significant amounts of propylene, propane, butenes, and butadiene.
During steam cracking, cracked gases emerging from the reactors are rapidly quenched to arrest undesirable secondary reactions which tend to destroy light olefins. The cooled gases are subsequently compressed and separated to recover the various olefins.
The recovery of the various olefin products is usually carried out by fractional distillation using a series of distillation steps to separate out the various components. Generally, one of two basic flow sequences is used. The two sequences are usually denominated as the front-end depropanizer sequence, commonly referred to as ‘front-end deprop’, or the front-end demethanizer sequence, commonly referred to as ‘front-end demeth’. Separation of the desired steam cracked olefin products from the overall product is known, and such separation processes do not form an aspect of this invention.
The manner in which the olefin stream to be purified by the invention is obtained is not critical to this invention, inasmuch as any hydrocarbon cracking method or dehydrogenation process typically forms an olefin stream that contains small amounts of impurities which can adversely affect further olefin processing such as polymerization.
For example, the separated olefins which are to be further used as a feed stream such as for polymerization, regardless of how formed, typically contain contaminants such as inorganic and organic sulfur-containing compounds, oxygenates, CO2 and water, which must be removed to levels below about 1 ppm to avoid catalyst contamination and consequent reduction in activity and/or selectivity in the downstream processing of the purified olefin stream. The terms “contaminant” and “impurities” are meant to be interchangeable and denote minor components such as above described, which have an adverse effect on the downstream processing of an olefin stream.
U.S. Pat. No. 6,403,854 (Miller et al.) discloses removal of an oxygenate contaminant such as dimethyl ether from an olefin stream made by contacting methanol with a silicoaluminophosphate (SAPO) catalyst. The oxygenate contaminant is removed by cooling the olefin stream in a two stage quench process. In the first stage of the process, a substantial portion of the dimethyl ether is removed along with condensed water as a bottoms product. Additional dimethyl ether is removed in the second stage, and the olefin overhead is further treated for oxygenate removal by contacting with an adsorbent.
U.S. Pat. No. 7,326,821 discloses a highly efficient and relatively simple process for removing oxygenates, particularly dimethyl ether or acetaldehyde, more particularly dimethyl ether, from an olefin stream. The process uses a solid adsorbent to remove a majority of the oxygenates from the olefin stream. The adsorbent can retain relatively large quantities of oxygenate, while being substantially inert to converting desired olefin product to undesirable by-product.
Desirably, the solid adsorbent is a molecular sieve or metal oxide. Preferably, the solid adsorbent is a molecular sieve. The molecular sieve preferably has a framework structure of at least 8 rings. Also preferably, the molecular sieve is a zeolite. Particularly preferred zeolites include zeolite X, zeolite Y, ZSM-5, ZSM-11, ZSM-14, ZSM-17, ZSM-18, ZSM-20, ZSM-31, ZSM-34, ZSM-41 or ZSM-46. Of these, zeolite X or Y is preferred, with zeolite X being particularly preferred.
The solid adsorbent can be kept in continuous use by regenerating the adsorbent following contact with the provided olefin stream. Regeneration of the solid adsorbent can be carried out by any conventional method. Such methods include treatment with a stream of a dry inert gas such as nitrogen at elevated temperature (temperature swing adsorption or TSA). In the regeneration stage, a regenerant comprising a hot fluid is passed along the flow path in a co-current, or more commonly, a countercurrent direction. The high temperature of the regenerant produces a desorption front in the bed which drives the sorbate from the sorbent material and into the flowing regenerant stream. This process continues until the bed is substantially sorbate-free, typically as indicated by the emergence of hot regenerant fluid at the bed exit.
In such TSA processes, the heated stream leaving the adsorbent, and which contains the desorbed contaminants is then either sent to a fuel header (if methane) or flare (if nitrogen). If nitrogen is used as the regeneration gas, then that nitrogen has a direct value, and if it were possible to recover a portion of that nitrogen, it would be valuable as nitrogen demand could be reduced by recycle of purified nitrogen back to the unit. Alternatively, if methane is used as the regeneration gas, there is value in purifying it also for reuse, as many steam cracking plants, especially those that use ethane as a primary feed component, do not generate sufficient amounts of methane for use in regenerating TSA purification systems.